Rocks Fall
When you see a gravel in an alluvial conglomerate, you might try to estimate the speed of river currents on the basis of the Shields’ diagram, but can you use the same approach to estimate the speed of sediment-gravity flow on the basis of the largest cla
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Rocks Fall
5.1
Sediment-Gravity Flows When you see a gravel in an alluvial conglomerate, you might try to estimate the speed of river currents on the basis of the Shields' diagram, but can you use the same approach to estimate the speed of sediment-gravity flow on the basis of the largest clast in a breccia formed by such a mechanism? The answer is no. What is a sediment-gravity flow? Almost all flows are produced by the effect of gravity, either directly or indirectly. In the case of fluid flows, or fluidgravity flows, the gravity acting on the fluid moves the fluid. We have seen that the moving force of a stream is a component of the weight of the moving water. In the case of sediment-gravity flows, the gravity acting on the sediment, or the weight of particles, angular fragments, boulders, pebbles, sand, or dust, moves the flowing mass of solid particles. This type of movement is familiar to skiers who are often witnesses of snow avalanches. An avalanche moves when a mass is separated from a cliff, falls down, and is broken into countless fragments or sediment particles of various size. The gravitational potential energy of the mass is changed into the kinetic energy of the motion, and the changing momentum of the motion is the force which compels the particles to move. Rock avalanche deposits, with debris flowing hundreds of kilometers, have been found on the Moon and Mars. The debris were carried neither by air nor by water; there is neither on those heavenly bodies. On Earth, an avalanche mass would soon be mixed with an ambient fluid, be it water or air. The movement of a solid-water mixture tumbling down a steep slope may, however, not be a liquid flow, because it is not necessarily the gravity acting on the water which makes the solid particles move. In very dense mixtures, it is the gravity acting on the sediment which makes the water move. Similarly, in the case of a solid-air mixture, such as a snow avalanche, it is the snow that moves air; snow is not carried by the air. When I was a young student working in the desert of western North America, one of our favorite pastimes was boulder-rolling. I remember particularly a steeply dipping bedding surface, exhumed by erosion, on the side of the Wasatch Mountains in Utah. We should have been doing field mapping, but often we K. J. Hsü, Physics of Sedimentology © Springer-Verlag Berlin Heidelberg 2004
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Chapter 5 . Rocks Fall
engaged ourselves in a contest to see whose boulder would go the farthest. Instinctively we all tried to find the heaviest boulder, but the biggest was not always the winner. In the downslope movement, the main resistance force is friction while air resistance is negligible; a well rounded cobble may have the least rolling friction. The physics of motion down an inclined plane is usually taught in middle school: At the moment when a sliding block begins to move, under its own weight, down an incline
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Fig. 5.1. Elm rockfall, 1882 (after Heim 1882). a A map showing the lines of movement of the Elm rockfall debris. Th
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